1. Field of the Invention
The present invention relates to a mask data creation method used for manufacturing a semiconductor device, especially a mask data creation method for a mask with auxiliary patterns.
2. Description of the Related Art
An patterns on a device have been becoming more and more miniaturized as density of a semiconductor device increases. For example, a dynamic random access memory (hereinafter referred to as a DRAM) with 1 gigabit of capacity has been made for practical use. In this sort of large capacity DRAM, width of a line and interval between lines, or line and space, and patterns have been miniaturized. A memory cell of the DRAM is composed of one transistor and one capacitor, and a predetermined number of the memory cells are arranged in X and Y directions according to a scale of integration to form a memory cell array area. Reading/writing information to/from the memory cells is performed by driving a circuit including word lines acting as gates to switching transistors and bit lines for reading and writing information to/from capacitors arranged around the memory cell array. The word lines and the bit lines are orthogonal to each other, and are provided as repetition of a pattern, which is made of lines and spaces between the lines.
In a DRAM, memory cells are allocated most densely in a memory cell array area in order to achieve a larger capacity in a limited chip area of the DRAM. As a result, word lines and bit lines are densely arranged in a pattern of repetition of the lines and intervals between the lines, or a line-and-space pattern, in which width of the lines becomes close to a resolution limit defined by a lithography process, which depends on a photolithography device, resist and so forth. Furthermore, circuits such as a sense amplifier, a decoder and the like, are arranged as peripheral circuits in the memory cell array area with the same pitch as the memory cells. Furthermore, on the further outside of the circuits, a control circuit and an input/output circuit are provided. There is a strong demand especially for a higher scale of integration for a DRAM, and effort has been made for achieving a resolution as high as possible.
Here, resolution means a ½ pitch of the smallest periodic pattern which can form a pattern with a practical depth of focus. The depth of focus means a range of focus which can form a pattern in a resist on a light exposure substrate. If the depth of focus is too small, light is not fully irradiated in a direction of film thickness because of unevenness of film thickness of the resist or a surface of the semiconductor, thus a prominent problem occurs in pattern forming. Therefore, it is desirable to use a large depth of focus.
Generally, it is known that a resolution R is proportional to a wavelength of a light exposure source λ, and inversely proportional to a numerical aperture NA, that is, R=K1*λ/NA. Here, K1 is a factor dependent on a process. Therefore, an improvement has been made in a photolithography device so that the wavelength of the light exposure source λ becomes shorter and the numerical aperture NA becomes higher, namely lower wavelength and higher NA. At present, KrF laser (248 nm of wavelength) and ArF laser (196 nm of wavelength) have been used as exposure sources. And, 0.8 and more for NA is obtained. By virtue of effect of other efforts, a critical size of 100 nm or less has now been achieved.
On the other hand, in lithography in which a mask pattern is transferred from the mask to the semiconductor substrate, it is an important subject to obtain both a smaller resolution and a depth of focus in light irradiation at the same time. It is represented that the depth of focus (DOF) is proportional to the wavelength λ, and inversely proportional to the square of the numerical aperture NA, that is, DOF=K2*λ/NA2. Here, K1 is a factor dependent on a process. Therefore, the depth of focus becomes smaller when a shorter wavelength and higher NA are obtained, and thus, it has been an important subject to secure a depth of focus.
As a method for securing the depth of focus, an oblique incidence illumination method such as zone illumination has been used. This is a method for irradiating only an oblique light to a mask by excluding a vertical light element from the light for illuminating the mask. A quadrupole illumination and the zone illumination have come into practical use. In a conventional usual illumination, image formation state by three-beam interference, in which three beams of zero order diffracted light and ± first order diffracted lights are collected with a projection lens, has been used. On the contrary, in the oblique incidence illumination, one of the ± first order diffracted lights is discarded (irradiated not to enter the projection lens), and two-beam interference, comprising the zero order diffracted light and the rest of the ± first order diffracted lights, is used to form an image.
Comparing the three-beam interference with the two-beam interference in the point of view from the best focus level of the image formation state, the intensity of the light of the two-beam interference is less than that of the three-beam interference since one of the ± first order diffracted light is discarded. However, when paying attention to a beam incidence angle on the image formation surface of the semiconductor substrate, the beam incidence angle for the image formation of the two-beam interference is ½ of that of the three-beam interference. Therefore, an image of the two-beam interference is less blurry when defocused, and sufficient light intensity distribution can be obtained when forming a resist pattern.
Furthermore, as another method, it has been known that it is possible to further increase the depth of focus by using a halftone phase shift mask. A halftone shift phase mask is one in which, a pattern of the mask in the shaded area is formed by a translucent pattern area, so that 2 to 20% of light leaks, is formed on a mask as a shaded area, and a phase of the penetrated light is inverted 180 degrees from a phase of the light of a transparent area adjacent thereto. In the dense pattern made of the line and space described above, the balance of the zero order diffracted light and one of the ± first order diffracted lights is ameliorated and the light intensity is improved when the halftone mask and the oblique incidence illumination are combined by making use of the diffracted light.
This sort of method shows a great effect when applied to the dense pattern made of the line and space described previously, however, the oblique incidence illumination method has little effect to an isolated pattern which does not originate the diffracted light, and thus increase of the depth of focus can not be expected. Here, the isolated pattern includes a case in which the interval between patterns is sufficiently large, and a case in which the interval is relatively close but not as dense as the dense pattern, that is, a middle pitch.
For increasing the depth of focus in the isolated pattern, on the contrary for the case of the dense pattern, decrease of the NA value (the light including only an almost vertical element is irradiated to the mask) and σ value is effective. Here, σ is called a coherence factor, and is represented by a ratio of NA of a lighting optical system and NA of a projection lens. This means that When σ is small, a small light close to a point light source illuminates the mask, and when σ is large, a large light source illuminates the mask. When the halftone phase shift mask is used, an illumination of small σ can also improve the depth of focus. This sort of condition for increasing of the depth of focus of the isolated pattern results in causing an ill effect of miniaturizing the critical size of the dense pattern made of the line and space. Therefore, it has been difficult to satisfy both exposure features of the miniaturized dense pattern and the isolated pattern.
Therefore, a method using a minute pattern called an auxiliary pattern, which do not serve for resolution itself, has been studied in order to satisfy both the dense pattern and the isolated pattern. The auxiliary pattern is also called a light intensity adjustment pattern, an assist bar, a scattered bar, an diffraction bar, or assist feature. By using the mask on which the auxiliary pattern is allocated adjacent to the isolation pattern under a condition of an oblique incident illumination, it is possible to obtain an image formation state which is close to the two-beam interference, and obtain an increased depth of focus. The location and the width of the auxiliary pattern affects the depth of focus of the transferred pattern.
The allocation of the auxiliary pattern is performed during the rule-based OPC process. The OPC stands for Optical Proximity Correction, and means correction in light proximity effect of lithography. Generally, the OPC method is roughly divided into a rule-based OPC and a model-based OPC. The rule-based OPC is a method for correcting a mask pattern based on an amount of correction of the size of a predetermined auxiliary pattern generating portion, by using parameters such as a width of the pattern which is a target for generating the auxiliary pattern, and an interval between the target pattern and a pattern near it, and the like.
On the other hand, the model-based OPC is a method for correcting a mask pattern so that a desired transferred pattern is obtained by using a light intensity simulation based on parameters such as an optical condition, a resist and the like. In order to create a model of the model-based OPC, it is necessary to change the pitch relative to each targeted size in the greater and smaller ranges including the latter, measure the size of the transferred pattern, and match the parameters to be suitable for the measurement result. Then, the light intensity distribution of each portion of the mask is calculated by using the deduced model. Subsequently, the limit of the light intensity by which the resist which has received the light is dissolved is used as a threshold, and correction of the mask size is performed so that a light intensity position of the designated threshold (corresponding to an edge position of a pattern transferred to the resist) becomes a desired size of the transferred pattern.
The reason why the allocation of the auxiliary pattern is carried out by using the rule-based method is that an effective position for allocating the auxiliary pattern is limited. That is, in order to form the image formation state of the two-beam interference, like the case of the dense pattern, the auxiliary pattern should be allocated in a position, adjacent to the pattern, with a fixed pitch interval in which the two-beam interference can occur. Therefore, the model-based OPC which determines an optimal position by performing a simulation for each pattern is not required. The rule-based OPC, which creates a rule table in advance and makes corrections according thereto, is more suitable since the process is speedy.
As described above, the auxiliary pattern can be relatively easily determined by selecting the location to form the two-beam interference between adjoining patterns and allocating so. However, the optimal size (width) of the auxiliary pattern largely depends on a process condition, it is not possible to decide it only by the optical condition. It is desirable that the width of the auxiliary pattern for obtaining an improved effect of the depth of focus is close to the upper limit of the transferred size, but not to be transferred.
However, since whether or not the auxiliary pattern is transferred is largely depend on a photosensitivity of the resist, the optimization of the width had to be experimentally decided by allocating a different width of the auxiliary pattern for each line pattern location. The process of optimizing the size is the most time-consuming part for creating a rule for the mask with the auxiliary pattern. That is, there is a plenty of steps for creating the mask rule.
In a lithography applied to a conventional semiconductor device with a loose rule, in which the smallest line width is on the order of 300 nm, taking into consideration of the dense pattern which is made of the line and space, and a sparse pattern in which adjacent patterns sufficiently distant from each other, it has been a large subject to study an allocation method of the auxiliary pattern for the sparse pattern. U.S. Pat. No. 5,242,770 and U.S. Pat. No. 5,447,810 disclose allocation methods relating to this.
However, a lithography applied to a semiconductor device with a current severe rule, such as a rule in which the smallest line width is on the order of 100 nm, a size correction of the adjoining pattern with the middle pitch, in which a pattern interval is relatively close but not as dense as the dense pattern, is an important subject, rather than the sparse pattern. Since the middle pitch is largely used for a peripheral circuit located around the periphery of the memory array described previously, the interval thereof is not fixed but varied. Conventionally, since a pattern width was wide, it has been possible to treat as a sparse pattern in which each of the adjoining patterns allocated in the peripheral circuit are not affected by each other, or the effect can be ignored.
However, nowadays, in order to meet a demand for improving a scale of integration, the pattern width became narrow, and each pattern has to be allocated close to each other, thus an effect among each pattern can not be ignored even in the peripheral circuit. As described above, it is necessary to study the optimal width of the auxiliary pattern for allocating thereof for using in the middle pitch pattern with various intervals. Therefore, it causes to require more time for a creating mask rule, and this enlarges the problem.
U.S. Pat. No. 5,821,014 discloses a method for allocating an auxiliary pattern having a different width case by case in order to allocate the auxiliary pattern to various pitch patterns. In this disclosure, the auxiliary pattern allocation is examined by matching the auxiliary patterns with different width until the optimal pattern is obtained, therefore mask creation time is increased as described above. Furthermore, Japanese Unexamined Patent Publication No. H6-242594 discloses an allocation method for an auxiliary pattern for a middle pitch pattern. However, it does not mention difficulty for deciding width of the auxiliary pattern to be located for various middle pitches.
As described above, conventionally, three parameters, that is, the location, the number, and the width of the auxiliary pattern have been decided by using the rule-based OPC. The location of the auxiliary pattern, and the number of the auxiliary pattern can be set easily by using an optical condition and a predetermined rule, respectively. However, as a circuit becomes miniaturized, it has been required greater amount of time for deciding the width of the auxiliary pattern to handle patterns with various width and intervals. According to miniaturization of the pattern, pattern correction has been necessary even for the middle pitch pattern on a mask, which had been treated as a sparse pattern. Various sizes are used in the middle pitch pattern, and thus a problem that a great amount of time is required for deciding a rule in a rule-based OPC occurs.